The present invention is directed to a work vehicle with a speed change arrangement such as an agricultural or construction vehicle.
FIGS. 1, 2, and 3 of the Publication of Japanese Patent Application No. H3-163263 discloses a work vehicle equipped with a rotational speed sensor for the engine with which a standard rotational speed is set when the engine is under no load. The speed change device is automatically shifted to either a higher speed position or a lower speed position depending on the actual engine rotational speed and the standard rotational speed.
The speed change device of the vehicle is automatically shifted to a higher speed position when it is decided that the load on the engine is small when the difference between a set rotational engine speed set by an accelerator control member (such as a manually operated accelerator lever or an accelerator pedal) and the actual engine rotational speed is small. The speed change device is automatically shifted to a lower speed position when it is decided that the load on the engine is large when the difference between a set rotational engine speed and the actual engine rotational speed is large.
Also, there are work vehicles where their speed change devices are automatically shifted either to a higher speed position or a lower speed position based on the amount of load on the engine. For example, in the work vehicle disclosed in the above mentioned publication, the speed change device is shifted to a lower speed position when the engine rotational speed decreases to a lower threshold and stays below the threshold for a predetermined length of time to avoid an excess engine load.
To address a problem of frequently changing engine rotational speed, the publication of Japanese patent application NO. H10-248308 also discloses a vehicle where the speed change device is shifted to a lower speed position not immediately after the actual engine rotational speed reaches a lower threshold but only after the rotational speed stays below the threshold for a predetermined length of time so that the speed change device would not be shifted to a lower speed position too frequently.
Work vehicles run under different running conditions including a work run where the vehicle runs substantially at a constant speed (so that the accelerator is not operated very frequently) with an implement in operation, and a on-road traveling condition where the accelerator may be operated more frequently. Therefore, improvements can be made in the area of automatically shifting the change speed device to address the diverse conditions that work vehicles experience.